As the digital audio and video technologies progress and numerous mobile communication products are developed, the transmission speed and the amount of data transmitted using wireless communication are both increasing. For the related applications, the IEEE 802.15 Wireless Personal Area Network (WPAN) standard designates an ultra-wide band of 3.1 to 10.6 GHz for high data-rate transmission to meet the needs of future technologies. In the antenna design for this kind of ultra-wideband antenna, it is highly valuable to obtain a design that has an operational bandwidth with a frequency ratio exceeding 1:7.5. Most of the current wideband antennas are monopole antenna, logarithm periodic antenna, and so on. However, these designs do not satisfy the ultra-wideband operation and are usually bulky in design.
In 1982, U.S. Pat. No. 4,466,003 disclosed a monopole antenna, including a plurality of metal rods having different lengths. The antenna can generate a plurality of resonant frequencies, but the disadvantages are that it has a complex structure and is large in size. In 1996, U.S. Pat. No. 5,828,340 disclosed a wideband monopole antenna. However, the increased bandwidth does not satisfy the needs nowadays.
FIG. 1 shows a schematic view of a conventional wideband monopole antenna. A wideband monopole antenna 100 uses a single feeding strip 130 to connect to a radiating metal plate 120. One end of feeding strip 130 is connected to a signal source (not shown) through a via-hole 140 on a ground plate 110. This type of wideband monopole antenna has the disadvantage of insufficient bandwidth, usually less than 2 GHz. Furthermore, for frequencies over the operating band, the cross-polarization radiation of the antenna is too large.